In automated processes or machines the controlling device monitors various sensors within the controlled process, evaluates the differences between the actual process state and the control algorithm requirements, and then activates appropriate process actuators. Conventionally, every automated system requires an individual electronic interface between a controller and each of the sensors and actuators of the process or machine being controlled. The sensing and actuating tasks are complex.
The sensor and actuator portion of the interface and interfacing task is complex and extensive at least in part because of the wide variety of voltages and currents such sensors and actuators encounter and therefore encode, interface with, and/or respond to. This problem became apparent at least since the 1969 development of the first general-purpose control device, the programmable logic controller (PLC). Conventionally, manufacturer's solution to the problem has been a series of compromises that do not favor the system designer or user.
There are four basic signal types for purpose of convenient description and analysis, they are: (1) analog inputs, (2) analog outputs, (3) digital inputs, and (4) digital outputs. The number of possible signal types is further compounded to include such characteristics as whether the signal is an ac (alternating current) signal or a dc (direct current) signal, whether it expresses logic levels (typically within the range of ±15 volts) or power line levels, whether the signal is fast or slow, and many ranges for analog sensors and actuators. Realistically there is a need to handle or interface over fifty different signal types for existing sensing, actuating, and control applications.
More usually, a manufacturer address the problem by limiting their standard products, for example, they may produce and market a subset of the ten to twenty interface blocks representing the most common signal types. This makes it necessary for the user requiring an interface circuit to use external signal conditioners to fill in voids for the less common signal types for which individualized blocks are not readily available commercially. Typically these plug-in signal conditioning blocks handle from one to sixteen I/O points of a single signal type.
This approach is not only expensive for the manufacturers to produce, document, market, stock, inventory and sell even the ten to twenty or so common different interface block types, but the designers who use them must select the appropriate type, and the controlled system user must maintain an inventory of essential spares so that spares are available for each type. The requirement to spare and service each type increases costs at the user end as well but not having an available spare of the correct type may mean shutting down a machine or process until a spare or repair is available, perhaps idling an assembly line and sending workers home at considerable cost.
The requirements for so many different signal interface block types also increase the risk that the wrong type will be used or will be miswired. This creates risk for personal and property damage.
There is also a need for a comprehensive interface component or circuit that provides for multiple analog voltage inputs that are switch or software/firmware programmable to support a range of analog voltage inputs extending from the micro-volt levels of thermocouple devices to high-voltage power ranges such as 220 VAC or higher.
Thus, there is a need for a universal or comprehensive interface that can handle more than one signal type or condition, or, where one cannot be designed or cost-effectively sold, for a fewer number of such comprehensive interface blocks that can handle classes or a plurality of signal types. There is also a need to provide such interfaces that reduce the risk of harm caused by miswiring. There is also a need for an interface that permits simultaneous sensing inputs and outputs.
There is also a need for a comprehensive interface component or circuit that provides combined inputs and outputs supporting these features rather than providing input and output in separate or piecemeal manner.
Thus, it is desirable to provide a comprehensive means to overcome the multiple deficiencies in order to simplify the design, manufacturing, and/or control of automated processes and machines.